Before 2009: very few adult skiers in Austria and Germany wore helmets, pretty much only the racers and children who were legally required to. I felt like a dork wearing mine, but I experienced a scary near-miss my first season on these relatively crowded slopes.
After 2009: at least 80% of adult skiers in Austria and Germany wear helmets, from the looks of things. Most frequent non-wearers are very old, permanently tanned Austrian men who literally have 5000+ ski days behind them or young ladies with nicely-done hair and make-up much more interested in getting good pictures for Instagram than actually skiing.
What happened in 2009? The Ministerpräsident (think US state governor) of Thuringia, Dieter Althaus, was flying down a black (expert) slope, turned onto a blue (beginner) slope with enough momentum behind him to go up it and crash into a lady from Slovakia.
He was in the hospital for weeks, living to earn a criminal judgment for negligent homicide, paying tens of thousands of Euro.
She died on the way to the hospital.
He was wearing a helmet.
She wasn't.
The absolute crush of helmet sales then and continued rates since are sometimes called the "Althaus-Effekt".